Variable displacement pumps, motors and pump-motors provide one or more of variation in flow and torque and hence, power, by changing the displacement of fluid within the pump, motor or pump-motor. A pump-motor refers to a fluidic machine that can operate as a pump or as a motor. In some examples, variable displacement pumps and motors and pump-motors include axial piston pumps and motors and pump-motors including a plurality of pistons slidably received in a corresponding plurality of cylinders. The plurality of pistons are coupled with an adjustable swash plate. As the angle of the swash plate is changed (e.g., from a measure of 0 degrees to 1 or more degrees) the pistons correspondingly increase their stroke and thereby displace larger volumes of fluid. In the case of a pump, the larger displacement moves a larger volume of fluid to the fluid system, which means that greater fluid flow and power is transferred from the prime mover into the fluid system. In the case of a motor, the larger displacement generates greater torque and transfers a greater amount of power from the fluid system into the rotating shaft and in the process utilizes more fluid from the fluid system. Each of the pistons are exposed to high pressure during the high pressure portions of the respective pumping and motoring strokes of the pistons even if the strokes (and corresponding displacements) are relatively small.
In other examples, variable displacement pumps and motors use one or more electrically operated and controlled stage valves for each piston and cylinder to control the opening of the cylinders to high pressure fluid over a variable portion of the piston stroke of each of the plurality of pistons. For instance, pairs of electrically operated check valves are used as the main stage valves or operate the main stage valves according to a displacement algorithm included in a logic controller for the pump or motor. The logic controller operates each of the electrically operated check valves in correspondence with portions of the piston strokes to realize a specified displacement and corresponding pump or motor performance.